1. FIELD OF INVENTION
This invention pertains to plating, and more particularly to depositing metal oxides by a method involving release of ammonia from ammine complexes.
2. Description of the Related Art
Plating technology is widely used in modern society. Typical examples include gold-plated jewelry, silver-plated dinnerware, chrome-plated automobile parts, copper-plated pots, and zinc-plated steel. Plating is also used for a variety of processes such as corrosion resistance, surface hardening, and appearance. As described in detail below, plating is widely thought to have present and future applications in the use of high-temperature superconducting materials. In addition to plating, many other techniques for coating surfaces are also known, including sputtering and anodizing.
While many methods are known for plating with electrically conductive substances, such as gold and silver, very few methods are known for plating or depositing substances, such as metal oxides, which are nonconductive under plating conditions. Using the traditional plating techniques commonly used today, parts to be plated must be electrically conductive, or they must be able to withstand high temperatures. Most contemporary plating technology is therefore limited with regard to the types of substances that can be successfully used as plates or substrates.
For most of this century, the flow of electric current without resistance was thought to be confined to metals cooled to temperatures near absolute zero. Such resistance-free flow of electricity, termed "superconductivity", is presently conceived as promising civilization significant technological advances. A finding of great significance is the recent discovery of substances that become superconducting at temperatures much higher than earlier thought possible.
During the last decade, it was discovered in Europe that certain metal oxides, or oxide compositions, become superconducting at temperatures substantially above absolute zero. These substances exhibit superconductivity when cooled to temperatures above the boiling point of nitrogen. Since liquid nitrogen is available in bulk at prices not much different from those of common liquids, such as milk, superconducting substances promise to have household applications. Even though the temperatures at which such substances operate are very cold by the standards of everyday life, these oxides are called "high-temperature superconductors." Since the original European discovery, many new high-temperature superconductors have been discovered. Most of them are based on metal oxides or on mixtures of metal oxides.
In order to take advantage of the new superconducting oxides, it will be necessary to obtain them in customary shapes of electrical conduits: wires, bars, and cables, for example. Metal oxides, however, are not usually flexible; they are typically brittle, crumbly, ceramic-like substances. This fact presents technical problems, since electronics often depend on the flexible nature, of wires for such purposes as windings and odd-shaped conductors to transport electricity from one location to another in complicated electrical machinery.
The problem of using brittle ceramics for the superconduction of electricity has many possible solutions. One of these is the depositing of superconductors on the surfaces of nonconducting substrates. Deposition of superconducting oxides in the form of continuous deposits of thin films in patterns that mimic complicated wiring diagrams will solve problems otherwise associated with the brittleness of the ceramics and will significantly advance the practical application of superconducting substances.
The importance of thin-film technology to application of superconductivity is summarized in Science, 241 (1988) 163. According to the article, superconducting thin films will be essential to the practical application of superconductivity in microelectronics. Problems now associated with techniques for depositing metal oxides, however, are significant. For example, some existing techniques for deposition of thin films of superconducting oxides require that the substrate onto which the metal oxide is to be plated be heated to high temperatures; in some cases, those temperatures are high enough to cause degradation of the substrate and superconducting oxide or reaction of the metal oxides with the substrate. In addition, high-temperature processes can also cause rough surfaces or cracks in the metal oxide film, which may render otherwise superconducting circuit designs useless. Moreover, the films deposited by existing techniques may be very thin, and thus have limited current-carrying; capacity. For these reasons, as well as many others, substantial improvements in deposition technology for metal oxides are greatly needed.
The phenomenon of precipitation from homogeneous solution has been known for many years. In this phenomenon, precipitate is formed in solution by a slow chemical reaction. Precipitates formed under these conditions are apt to be rather pure, because precipitation from homogeneous solution minimizes the coprecipitation of undesired substances. Precipitation from homogeneous solution is used, for example, in gravimetric quantitative chemical analysis, where the weight of a dried, highly pure precipitate is the critical factor in ensuring the accuracy of the analysis. Some precipitates that are formed slowly, as in the homogeneous precipitation process, can be deposited directly on some inert substances, such as glass. In the present invention, the slowly-formed precipitates of metal hydroxides do not occur as suspensions of solids, as in usual precipitation processes, but rather form coatings (thin films) on substrates such as glass.
It is well-known that many metals form ammine complexes in solution. These complexes are written as M(NH.sub.3).sub.n, where "n" is the number of ammonia molecules in the complex. Typically, "n" is an integer, often 2, 4, or 6. The complex will typically be charged, although in the above representation, the charge was omitted. A typical ammine complex is the blue species formed by copper ions and ammonia: Cu(NH.sub.3).sub.4.sup.2+.
Complexes, including ammine complexes, "stabilize" metal ions and can be used to prevent their precipitation. Thus, a metal cation that is not complexed will form a precipitate upon coming into contact with a precipitate-forming anion. In the presence of a complexing agent, however, the precipitate-forming anion can be prevented from precipitating the metal. Both precipitate-forming anions and complexing agents compete for the metal ions. If one of these species is more successful than the other in reacting with the metal ions, it will dominate the reaction, thus either causing or preventing precipitation. Calculations involving solubility products and the stability constants of the metal ammine complexes can be used to delineate (in an approximate manner) the range of metal and hydroxide ion concentrations that are appropriate for the process of the present invention.